In the course of Bayer alum earth processing technology the alum earth containing mineral is mixed with sodium hydroxide and the alum earth is digested using a line consisting of preheaters and/or autoclaves, or in tube digesting equipment. Digesting equipment consisting of preheaters and autoclaves is disclosed in the Hungarian Pat. No. 149 514 and in the German Pat. No. 1 920 222. Upon heating the slurry in these types of equipment, at the slurry-side of the boiler tubes a deposit forms which must be removed from time to time. In case of equipment consisting of preheaters and autoclaves cleaning is performed by cyclic cut off of each unit, sometimes by cyclic putting out of each line, while for the mentioned tube digesting equipment cleaning is carried out by disconnecting one or more boiler tubes. Since frequent cleaning is inevitable for the efficient operation of the equipment, the possibility and manner of cleaning indicate up-to-dateness of the equipment.
The chemical composition of the deposit depends on the chemical composition of the alum earth containing mineral, but the amount of deposit is mainly determined by the silicates because of the silica which is always present in the starting mineral. In the methods disclosed in the above-mentioned patents the deposits are removed with the aid of a cleaning fluid, which method requires supplementary equipment at additional cost.